Considering the reflection of the Sun's light off the Earth,
we can estimate their relative brightness as

                                                      2
         brightness of Earth        ( 6.37 x 10^3 km ) 
    ---------------------------  =  ( ------------   )  
         brightness of Sun          ( 1.50 x 10^8 km )


                                             -9
                                 =   1.8 x 10